why is it bad to replace ATF.??

whats up yotatech, why isn't it a good idea to replace ATF if the fluid in the resevoir is old? can some explain this to me. also, My rig has 148xxxxxx Miles on the odometer with no knowledge of knowing if the ATF fluid has been changed or not. I will be replacing my tranmission pan gasket, when taking off the pan to replace the gasket I know that I will loose some ATF, with that in mind, should I perform a full ATF flush or just pour in more ATF into the dipstick HOLE to compensate for what has been lost..? Thanks.

whats up yotatech, why isn't it a good idea to replace ATF if the fluid in the resevoir is old? can some explain this to me. also, My rig has 148xxxxxx Miles on the odometer with no knowledge of knowing if the ATF fluid has been changed or not. I will be replacing my tranmission pan gasket, when taking off the pan to replace the gasket I know that I will loose some ATF, with that in mind, should I perform a full ATF flush or just pour in more ATF into the dipstick HOLE to compensate for what has been lost..? Thanks.

Not an expert in this area but read a ton before changing ATF on my truck. The rationale of not doing a flush at high mileage when it hasn't been done before is the flush will accelerate an already bad situation with the tranny. It appears that your old, burnt fluid can possibly be holding pieces of the tranny together, and by replacing it w/ fresh fluid can result in total tranny failure.

If you search or just ask, others will chime in on how to identify burnt fluid. I have a 3rd gen and don't believe tranny fluid was ever changed before 90k. I decided to drop the pan, clean the magnets and replace the fluid from the pan. I also flushed the ATF in the cooler but left the remaining ATF in the torque converter and lines. It appears that my fluid wasn't in that bad a shape so I may do a drain and refill soon but won't be doing a flush. Hope this helps.

Location: 100 miles offshore as much as possible, & Springfield Oregon USA

Posts: 3,291

It is certainly better to replace the fluid and filters more regularly, but I have never heard an actual transmission mechanic or manufacturer suggest that after the fluid has seen extended service to leave it in there till the transmission fails. Changing the fluid and filter is ALWAys better than just giving up and leaving it there. IT's no different than engine oil. If you buy a used vehicle and the oil is really dark and ugly, would you just leave it in there and drive it till it dies? Of course not.

Simple test to see if it's overheated - rub it between your fingers and sniff it. If it's been overheated it will smell "burnt." It's distinct.

The power flush machine is better in that you get more fluid out but if old ATF has oxidized badly and is more like a sticky goo than oil, flushing it can flush it into spots where it may block flow and that it is typically how an ATF dies.

Just drop the pan and change it the old fashioned way.

It is generally accepted that if you vehicle has over 60k and never had a tranmission fluid change, do not use a powerflush machine. An old fashoned fluid change will only get about 75% of the old fluid out but thats better than nothing.

Once a fluid starts to quit, more slipping of the clutches will occur and fluid will heat rapidly and eventually actually start to coat the clutch plates ab brake bands etc. New fluid, even only 75% will not.

Some folks like to do it twice, change one and run a few hundred mils to flush, then drain and put in the fluid you wish to keep.

You may want to look into fluids designed for higher mileage transmissions. They have seal conditioners to control leaks and also are a higher initial viscoisty for better metal to metal proection.

and if you do it yourself, check on here or a FSM for the correct amount of quarts to refill with....i followed the haynes and was stranded on the side of the road with an overheating tranny short of 2 quarts

you shoud just through it in the gutter and buy an other......my truck if fixed Vee.....i wont be going to big bear this weekend.....i want to get those sliders on mang.....hey ask everyone if it would be better to convert to 4wd with IFS or solid axle.....everyone i asked said if it auto trans go solid axle, cause you will need a whole new tranny if you want IFS.....it way cheeper than IFS and less parts to f up.....people are changing to solid all the time....no IFS mang

ok, so when replacing my transmission gasket, do I compensate for the lost fluid in the pan or not.??should I pour more in? is it OK to mix old with new tranny fluid.? sorry for the noob Q's I am new to this tranny stuff.

Just me personally, but I'd flush it. But please don't take my advice over someone elses. I just don't see the point in dropping the pain to drain all of 3 quarts from the tranny, pouring new oil into the system only to have it immediately contaminated by the xx quares of old crap. But that's just me.

Location: 100 miles offshore as much as possible, & Springfield Oregon USA

Posts: 3,291

Agreed. You have to drain the torque converter too. Some old fluid always stays in there - same as your engine when you change the oil. It won't hurt anything.

You don't "compensate for the lost fluid in the pan" or anything like that. You make sure when you're all done that the right amount of fluid is in the trans - that's why they provide a dipstick. Here's a simple way to guesstimate the initial fill - keep track of how much comes out, then put that much back in. I pour mine into jugs (either gallon milk jugs or 5qt oil jugs) to take to the recyclers, so it's easy to total up how much I drained. Put that much in, warm it up, and check the dipstick.

I just bought a 99 4Runner 2wd and I took it to the stealer to get a tranny flush. The fluid in it seemed ok, a little dirty, but I've seen worse. I figured if it had never been changed 60,000 miles would be a good time to get all new fluid in there. Was this a bad move? I haven't really noticed any difference in it at all. Why are you guys against the total flush?

With 150K mile truck and no history on it, I would simply do drain/replace... not once though,
but twice, one shortly after another.
I did flushed my tranny once, just after I bought my truck, but that was at about 25K miles.
Soon after that, I installed tranny cooler and filter and replaced the ATF with synthetic.
Since then, I just simply replace the ATF once a year, and every time the used fluid looks almost like new.
The tranny is happy and so am I.

Agreed. You have to drain the torque converter too. Some old fluid always stays in there - same as your engine when you change the oil. It won't hurt anything.

You don't "compensate for the lost fluid in the pan" or anything like that. You make sure when you're all done that the right amount of fluid is in the trans - that's why they provide a dipstick. Here's a simple way to guesstimate the initial fill - keep track of how much comes out, then put that much back in. I pour mine into jugs (either gallon milk jugs or 5qt oil jugs) to take to the recyclers, so it's easy to total up how much I drained. Put that much in, warm it up, and check the dipstick.

Just me personally, but I'd flush it. But please don't take my advice over someone elses. I just don't see the point in dropping the pain to drain all of 3 quarts from the tranny, pouring new oil into the system only to have it immediately contaminated by the xx quares of old crap. But that's just me.

Old oil will not contaminate new oil other than mixing in with it. The viscosity of the old fluid is likely a bit higher if severly oxidized and also it may be neutral to acidic, again if severly oxidized. The new fluid will neutralize the acidity. The new fluid will restore the correct co-efficients of friction and add anti wear and detergents and dispersants and seal conditioners ... none of which will be affected by the old fluid.

100% of reputable shops will make you sign a waiver if you want a powerflush on an older vehicle.

Again, you call and you could be lucky but based from where I sit, in an industry that has day to day involvment with ATF's, if a quick lube center has a tranny failure on a customer vehicle that they powerflushed and it has over 60k on it , they honor their own warranty, not the oil supplier.

i disagree with the part about not using a flush machine. get that old fluid out of there ASAP, using whatever method is available. many flush machines use the tranny's pump to actually pump the fluid, so it's not going to cause any problems by having too much/too little pressure while it's flushing.

I have an older runner and was following the post as I was doing my own investigating of whether or not to flush. I called around to several garages/shops and nobody seemed to want to drop the pan AND flush, so i have decided based on a conversation with a transmission shop owner to just drop the pan, drain and fill on my own.

I also found out thanks to fill that my 4Runner does not have a filter but rather a screen. I guess I"m just gonna check the screen rather than replace because it is $$$$$$

I have an older runner and was following the post as I was doing my own investigating of whether or not to flush. I called around to several garages/shops and nobody seemed to want to drop the pan AND flush, so i have decided based on a conversation with a transmission shop owner to just drop the pan, drain and fill on my own.

I also found out thanks to fill that my 4Runner does not have a filter but rather a screen. I guess I"m just gonna check the screen rather than replace because it is $$$$$$

I was in the same boat as you and really didn't want to encounter an issue w/ the tranny. I also did the same thing w/ my old Accord and was talked into letting Jiffy Lube do a tranny flush at 90k. I remember how crappy the Accord ran after the flush and after asking several people, some of the reasons above are supposedly why the tranny started acting funny.

After dropping the pan on my 3rd gen, the tranny fluid was black but not burnt. My mechanic cleaned the magnets in the pan and refilled. After a week, we dropped the pan again and refilled. I know that this doesn't get all the old fluid out but I figured mixing in new synthetic would be better than nothing. I'm not an expert but this is what I did.